This invention relates to zinc sulfide-based electroluminescent phosphors. More specifically, it relates to orange-yellow-emitting zinc sulfide-based electroluminescent phosphors co-activated with manganese and copper.
Orange-yellow-emitting zinc sulfide electroluminescent phosphors co-activated with manganese and copper ions (ZnS:Mn,Cu) are well known. Examples of these phosphors and their methods of manufacture are described in U.S. Pat. Nos. 4,859,361 and 5,009,808. These phosphors are significantly lower in brightness than other ZnS-based phosphors. For example, the brightness of commercially available Zn:Mn,Cu phosphors is under 10 foot-Lamberts (ft-L) whereas the brightness of commercial blue-green emitting ZnS:Cu,Cl electroluminescent phosphors is greater than 30 ft-L.
U.S. Pat. No. 5,702,643 and JP 4-270780 describe improving the half-life of copper-activated zinc sulfide (ZnS:Cu) phosphors by incorporating small amounts of gold into the phosphor. U.S. Pat. No. 6,395,196 describes increasing the half-life of a ZnS:Cu electroluminescent phosphor by heating the finished phosphor in a closed vessel in the presence of antimony vapor. However, these techniques are insufficient to achieve the brightness levels needed to meet current market requirements. In particular, higher brightness orange-yellow-emitting electroluminescent phosphors are needed for forming white-emitting phosphor blends which could be used in electroluminescent lamps for backlighting color LCD displays. Therefore, it is desirable to improve the brightness of orange-yellow-emitting zinc sulfide electroluminescent phosphors.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a high brightness orange-yellow-emitting electroluminescent phosphor.
It is a further object of the invention to provide a method of making a high-brightness orange-yellow-emitting electroluminescent phosphor.
In one aspect of the invention, an orange-yellow-emitting zinc sulfide-based electroluminescent phosphor is provided which has a brightness greater than 10 foot-Lamberts and an x color coordinate from about 0.51 to about 0.56 and a y color coordinate from about 0.42 to about 0.48.
In another aspect of the invention, the orange-yellow-emitting electroluminescent phosphor comprises zinc sulfide activated with copper, manganese, chlorine, and a metal selected from gold and antimony, wherein the phosphor contains about 0.5 to about 1.3 weight percent manganese, about 0.02 to about 0.08 weight percent copper, about 0.002 to about 0.02 weight percent chlorine, 0 to about 0.012 weight percent gold, and 0 to about 0.0007 weight percent antimony.
In yet another aspect of the invention, a method of making an orange-yellow-emitting electroluminescent phosphor is provided wherein the method comprises:
(a) forming a mixture of an amount of zinc sulfide, an amount of a copper source, an amount of a manganese source, an amount of zinc oxide, an amount of sulfur, an amount of a chloride-containing flux, and an amount of a source of a metal selected from a gold and antimony;
(b) firing the mixture at a temperature from about 1100xc2x0 C. to about 1250xc2x0 C. for about 3 hours to about 5 hours;
(c) mechanically working the fired material to introduce defects in the crystal structure;
(d) washing the fired mixture to remove the chloride-containing flux and copper sulfide residues;
(e) combining the fired material with an amount of a copper source, an amount of a manganese source, and an amount of zinc oxide to form a second mixture;
(f) firing the second mixture at a temperature from about 750xc2x0 C. to about 950xc2x0 C. for about 2 hours to about 5 hours and fast cooling the fired second mixture; and
(g) washing the second mixture to remove residual copper sulfide.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
A high-brightness orange-yellow-emitting electroluminescent phosphor has been invented. The phosphor comprises zinc sulfide activated with copper, manganese, chlorine, and a metal selected from gold and antimony. The phosphor exhibits a brightness of greater than 10 foot-Lamberts (ft-L), and preferably at least about 13 ft-L, when stimulated in a conventional thick-film electroluminescent lamp. The orange-yellow emission has an x color coordinate from about 0.51 to about 0.56 and a y color coordinate from about 0.42 to about 0.48.
The phosphor preferably contains from about 0.5 to about 1.3 weight percent manganese, about 0.02 to about 0.08 weight percent copper, about 0.002 to about 0.02 weight percent chlorine, 0 to about 0.012 weight percent gold, and 0 to about 0.0007 weight percent antimony. A more preferred range for the gold content of the phosphor is from about 0.002 to about 0.012 weight percent gold and a more preferred range for the antimony content is from greater than 0 to about 0.0007 weight percent antimony. It should be noted that the amount of antimony retained in the finished phosphor may be very difficult to detect. In these cases, it is assumed that because antimony was added during the first firing step some antimony is retained even though it could not be detected in the chemical analysis.
The high-brightness phosphor of this invention is made in two firing steps. In the first firing step, zinc sulfide (ZnS) is blended with appropriate amounts of a source of copper (Cu), zinc oxide (ZnO), sulfur (S), a chloride-containing flux, and a source of a metal selected from gold and antimony. Preferably, the Au source is a pre-mixture of gold chloride (AuCl3) and ZnS, the Cu source is anhydrous copper sulfate (CuSO4), and the Sb source is a pre-mixture of antimony oxide (Sb2O3) and ZnS. The chloride-containing flux can be a mixture of alkali metal and alkaline earth chlorides, preferably barium chloride (BaCl2), magnesium chloride (MgCl2), and sodium chloride (NaCl). The blended mixture preferably contains in weight percent (wt. %) relative to the weight of ZnS: 0 to 0.018 wt. % Au, 0.02 to 0.08 wt. % Cu, 0 to 0.01 wt. % Sb, 0.3 to 0.7 wt. % ZnO, 6 to 12 wt. % sulfur, and 4 to 14 wt. % chloride flux (preferably 0-4 wt. % of barium chloride, 1-5 wt. % of magnesium chloride, and 1-5 wt. % of sodium chloride). More preferably, the blended mixture contains 0.008 to 0.18 wt. % Au. Even more preferably, the blended mixture contains 0.001 to 0.01 wt. % Sb.
The blended mixture is fired in air at a temperature from about 1100xc2x0 C. to about 1250xc2x0 C. for about 3 to about 5 hours. A fast cooling after the first firing is preferred but is not critical. The fast cooling is achieved by placing the red hot crucible into a water bath. Water is kept running to maintain the water bath at  less than 60xc2x0 C. The fired material is then water washed, dried, and gently mulled (low-intensity milling) to induce defects in its crystal structure. The mulling time depends on the particular type of equipment used and the amount of material being mulled. An optimum mulling time can be readily determined by one skilled in the art of electroluminescent phosphors. In our case, a typical mulling time was 75 minutes for 500 to 550 g of material.
After mulling, the material is washed with acid and then a basic solution containing sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and a chelating agent, such as diethylenetriaminepentaacetic acid (DTPA). In a preferred method, the basic solution contains relative to the phosphor weight: 2-4 wt. % DTPA, 2.5-3.5 wt. % NaOH, and 5-15 wt. % of a 30% H2O2 solution. This chemical wash further removes flux residues and copper sulfides from the phosphor surface. A KCN solution may be used in place of the basic solution to remove the copper sulfide residue. The material is then washed with hot deionized water and then dried to complete the first firing step.
In the second firing step, the material from the first firing step is blended with appropriate amounts of a copper source, a manganese source, and zinc oxide. Preferably, the material from the first firing step is blended with 0.2-0.8 wt. % anhydrous copper sulfate (CuSO4), 2-10 wt. % manganese carbonate (MnCO3), and 5-15 wt. % zinc oxide (ZnO) The blended material may then be fired once in air at a temperature from about 750xc2x0 C. to about 950xc2x0 C. for about 2 to about 5 hours or fired twice in the same temperature range first for about 1 to about 3 hours and then for about 0.5 to about 2 hours. It is preferred that the fired material be fast cooled since a slow cooling rate will degrade brightness. If the second firing is done on two stages, then the fast cooling is used after the second stage. As used herein, fast cooling means that the material is cooled to below about 200xc2x0 C. in less than about one hour. Slow cooling means that material is cooled to below about 200xc2x0 C. in more than 2 hours. The fired material is washed with hot deionized water, acid, and the basic solution of DTPA-NaOH-H2O2 used in the first firing step. After a final water washing to remove any remaining chemical residues, the material is dried and sifted to form a high-brightness orange-yellow-emitting electroluminescent phosphor. The finished phosphor typically has a particle size between 18 to 28 xcexcm.
Several examples of the high-brightness orange-yellow-emitting phosphor are shown below. All phosphors were tested in conventional thick-film electroluminescent lamps operated at 100 V and 400 Hz in a 50% R.H, 70xc2x0 F. environment. The test lamps are comprised of a xcx9c40 xcexcm-thick phosphor layer and an approximately 26 xcexcm-thick barium titanate dielectric layer. The lamps are constructed by combining the phosphor with a cyanoresin binder (Shin Etsu Co.) which has been dissolved in a mixture of acetone and dimethylformamide. In particular, the binder is made by mixing 575 g of acetone, 575 g of dimethylformamide, and 400 g of cyanoresin. The percentage of phosphor in the liquid binder is 75 wt. % and the percentage of phosphor after the binder-phosphor mix is dried is 80 wt. %. The phosphor suspension is blade coated onto a 0.007-0.0075 in.-thick PET film having a transparent, conductive layer of indium-tin oxide (ITO) (available from CPFilms). After drying, the barium titanate layer is applied over the phosphor layer in the same way using a suspension of barium titantate dispersed in the cyanoresin binder. In particular, the binder-barium titanate mix is made by mixing 375 g of cyanoresin binder, 375 g of barium titanate, and 82.5 g of dimethylformamide. The percentage of barium titanate in the binder is 45 wt. % and the percentage of barium titanate in the binder after drying is 80 wt. %. A rear electrode comprised of a 50 to 80 xcexcm-thick graphite layer is applied to the dried barium titanate dielectric layer using a graphite suspension (Acheson Colloids). Lead wires are attached and the entire lamp is laminated with a clear, flexible film (Aclam TC200 from Honeywell Corp.) which is applied to both sides. The lamps were operated from 24 hours prior to measuring their brightness in order to stabilize the lamps and obtain representative measurements. Brightness as used herein means the brightness of the phosphor in a conventional thick-film electroluminescent lamp which has been operated at 100 V and 400 Hz for 24 hours.